Traversing a molten zone in a crystalline bar by direct current reversal



United States Patent 3,453,088 TRAVERSING A MOLTEN ZONE IN A CRYSTAL- LINE BAR BY DIRECT CURRENT REVERSAL Alexander Lebelr, Berlin-Weissensee, and Siegfried Raab, Berlin-Johannisthal, Germany, assignors to Deutsche Akademie der Wissenschaften Zu Berlin, Berlin-Adlershof, Germany No Drawing. Filed June 14, 1965, Ser. No. 463,945 Int. Cl. B01j 17/20 U.S. Cl. 23-301 3 Claims ABSTRACT OF THE DISCLOSURE Direct current is passed through a polycrystalline bar in which a molten Zone is established in the bar. Due to Peltier effect, heating and cooling zones are establishedf at respective solid-liquid interfaces to advance the melt zone. A reversal of the direct current reverses the direction of the melt zone.

This invention relates to the production of single crystals and in particular for crystals produced from semiconductor materials.

In the production of doped and undoped single crystals, particularly from semi-conductor materials either by the Czochralsky process, vertical zone melting process, or horizontal zone melting process, a seeding crystal is employed. The seeding crystal is a crystal grown to a desired crystallographic orientation and has the function of providing the desired crystallographic order for the crystal to be grown by the above-named process.

In the known processes a crucible is employed wherein the material is maintained in a molten condition. The seeding crystal is lowered with one end into the molten metal, while the other end is held outside in such a manner that the seeding crystal is slowly withdrawn. By maintaining the temperature conditions properly and by cooling the mounted end of the seeded crystal, a single crystal of the same molecular pattern as the seed grows onto the seed until all of the material is grown.

In the zone-floating process the production of the single crystals is usually combined with the purification process so that a bar shaped material is formed, fused to the seed crystal and after repeated floating the polycrystalline material grows in the direction of movement of the molten zone leading away from the seed, thereby producing a single crystal. The production of the seeding crystals requires special care as they influence the quality of the single crystals to be grown thereon. It is, therefore, important that the seeding crystal produced be of high crystalline quality, free or nearly free of dislocations and of high purity. Additionally, special care must be applied to the accretion to the melt from which the crystals are produced. Normally, severe technical measures are required to maintain quality. Additionally, the formation of single crystals from seeding crystals is greatly 'infiuenced by external factors, e.g. voltage variations, and other conditions of the equipment employed in the process. Therefore, additional technical measures must be implemented in order to maintain the quality of the crystals produced.

It is, therefore, an object of the present invention to eliminate the above-named disadvantages for single crystal production.

It is another object of the present invention to provide an improved process for the production of single crystals.

Another object of the present invention is to provide a process for the production of single crystals directly from a polycrystalline material without using seeding crystals.

According to the invention, it is possible to produce 3,453,088 Patented July 1, 1969 ice single crystals from polycrystalline materials with or without a crucible process for the production of doped and undoped single crystals by employing the anisotropy of the Peltier or the Peltier and Thomson effect. In either lof these processes, a direct current is applied to a bar of the material. The current is controlled to correspond to the Peltier coefiicient or the Peltier and Thomson coefficients of the material. The current polarity is also reversed several times and/or the phase boundary comprised of the solid-to-liquid state is moved once or several times along the length of the bar. In the presence of at least two interfaces solid/liquid, liquid/ solid, e.g. in crucible free and not crucible free zone melting, the liquid zone (or liquid zones) are made to move through the bar once or several times. In this connection, reference may be had to our co-pending application Ser. No. 463,723.

The amount of heating or cooling applied at the solidto-liquid, or liquid-to-solid interfaces in accordance with the Peltier, or Peltier and Thomson effect, is dependent on the material. The following relationship applies for two conductors having the solid-to-liquid, or liquid-to-solid interfaces at a predetermined current direction,

for the Thomson effect; the signs of the Peltier and Thomson coefficients remaining the same.

Moreover, the Peltier and Thomson effects are highly dependent on the crystallographic orientation. For example, in silicon, the Peltier effect and also the Thomson effect in the [1-1-1] crystal direction is much greater than in the polycrystalline material which means that the heating or cooling at the solid-to-liquid, or liquid-to-solid interfaces in a certain direction of flow becomes considerably intersified in the [l-l-l] direction. This phenomenon is called the anisotropy of the Peltier respectively Thomson effect. Because of the anisotropy of the Peltier or Thomson effect, the crystallographic direction which has the greatest Peltier and Thomson effect occurs spontaneously.

By repeated movement of one phase boundary through its starting position when using the anisotropy of the Peltier or Peltier and Thomson effect, there is formed in the presence of the solid-to-liquid phase boundary a spontaneous adjustment of the crystallographic direction in which the Peltier and Thomson effect has its greatest control. By letting the liquid zones travel the length of the rod several times it has been discovered that the rod becomes mono-crystalline in the crystallographic direction in which the Peltier, or Peltier-Thomson effect has the greatest control over the material as a result of the anisotropy effect described above. Thus, by the application of the Peltier and the Thomson effect, single crystals can be produced without employing seeding crystals. This greatly simplifies single crystal production and eliminates many of the technical difficulties which affect the quality of the single crystals produced.

We claim:

1. Method for the direction production of monocrystals from polycrystalline material, which comprises passing a direct current of a magnitude corresponding to the Peltier coefficient through a bar of a polycrystalline semiconductor material having a melt zone bordered by solidliquid interfaces, thereby using the anisotropy of the Peltier effect to convert the crystalline structure of the material from polycrystalline to monocrystalline; and reversing the polarity of said current a plurality of times so as to cause the melt zone bordered by the solid-liquid interfaces to reverse direction and move along the length of the bar at each reversal of the polarity of the current,

3 thereby forming a zone of monocrystalline formation in the bar.

2. Method according to claim 1, wherein the magnitude of the direct current corresponds to the total of the Peltier and Thomson effects.

3. Method according to claim 2, wherein the Thomson effect is controlled by adjusted cooling of the end of the bar.

References Cited Lindenblad 623 Stanton 623 Newton 623 Jensen 23301 Davis 23301 Pfam 23301 NORMAN YUDKOFF, Primary Examiner.

G. P. HINES, Assistant Examiner.

US. Cl. X.R. 

